1. Field of the Invention
The present invention relates to a resonant switching power supply employing a parallel operation system in which a plurality of resonant converter circuits are connected in parallel.
2. Discussion of Background
A great deal of interest is focused on resonant switching power supplies having the potential to achieve high efficiency and low noise. A resonant switching power supply performs switching on a DC source through a switching circuit, causes the switching output to resonate at a resonant circuit and takes out the resonant output via the winding of a transformer to convert it to DC for output.
A resonant switching power supply may adopt a parallel operation system in which a plurality of resonant DC--DC converter circuits are connected in parallel in order to increase the power to be converted and to perform redundant operations.
As a means for achieving parallel operation by employing a plurality of resonant switching power supplies in the prior art, a plurality of switching power supplies are connected in parallel, the individual output currents are detected and control is performed independently for each of them to ensure that the output currents from the individual switching power supplies are almost equal to one another.
However, this type of parallel operation system, which requires a number of control circuits equal to the number of the switching power supplies, tends to increase the cost and leads to an increase in the size of the device.
In addition, the output stabilization control for a resonant switching power supply is achieved by controlling the switching frequency and changing the impedances of the resonant circuits. However, since the circuit constant values of a resonant capacitor and a resonant inductor in a resonant circuit are normally different among different resonant switching power supplies, the switching frequencies also vary among the individual resonant switching power supplies. Because of this, when equalization of the individual share of current for the individual switching power supplies is attempted, a frequency beat noise occurs, posing a problem in that the control becomes unstable.
If one control circuit is shared by a plurality of resonant switching power supplies, synchronized control can be performed at one frequency and, as a result, generation of frequency beat noise and unstable control can be avoided. However, in this case, zero volt switching (hereafter referred to as ZVS) becomes difficult. Namely, in a plurality of resonant switching power supplies that are operated in parallel, if the impedances of the resonant circuits are different from one another due to different constant values of the circuit elements and the like, the energy that has been accumulated in the resonant circuit may have already been discharged in a converter circuit with a high resonance frequency at its resonant circuit during dead time in which the switching elements are turned off, resulting in almost no current, which makes ZVS impossible.
U.S. Pat. No. 4,648,020 discloses a technology through which the constant values in the resonant circuits are matched in the individual switching power supplies to achieve balanced load sharing among the individual resonant switching power supplies. However, this technology requires matched selection of the circuit elements that are to constitute the resonant circuits, which causes a reduction in productivity and an imbalance in load sharing resulting from the different tolerance ranges. Similar problems occur due to changes that the circuit elements undergo over time and the like.